1. Field of the Invention
This invention relates to a machine tool and, more particularly, to a machine tool provided with an automatic tool changer (hereinafter referred to as ATC) that automatically changes tools according to requirements of an on-going machining process.
2. Prior Art
ATCs that are currently used for machine tools in machining facilities, such as machining centers, normally involve the use of a plurality of actuators for changing tools. However, such an arrangement has the drawback that a long time is required for changing tools because the overall configuration of the ATC is very complicated and bulky and requires an exchange many signals between any two successive machining operations. In view of this problem, Japanese Utility Model Application Laid-Open No. 63-113530 proposed the use of a pair of coupled cams for slewing the exchanger arm and pulling out/feeding in tools. Japanese Utility Model Application Laid-Open No. 63-133945 further proposed the use of an additional cam for opening and closing the arm claw and another additional cam for unclamping the spindle tool. Japanese Utility Model Publication No. 5-33242 has also disclosed the use of cams for opening and closing the ATC door in addition to swivelling the exchanger arm and pulling out/feeding in tools.
FIG. 1 schematically illustrates an ATC unit according to Japanese Patent Application Laid-Open No. 6-63833. The unit shown operates to clamp, unclamp or remove a tool. To clamp or unclamp a tool, a draw bar 107 is moved within a spindle 105 by a spring 106 and a cam 108, that is driven to rotate by a motor 100A by way of a drive shaft 109. In clamping, the draw bar 107 is urged into the spindle 105 by coned disc springs 106 and thus draws a tool T into the spindle 105 by counter-sunk springs 106 until the tool T is clamped within the spindle 105. To unclamp and replace the tool with another tool, the draw bar 107 is pushed out of the spindle by the cam 108 until the tool T is unclamped. The tool is then moved away by an exchanger arm 101 that is driven to operate in a controlled manner by a pair of cams 102, 103 that are driven by a motor 100B by way of a drive shaft 104.
In order for a tool changer to automatically change tools, the following five operations have to be carried out. (not in this order)
(1) The changer arm pulls the existing tool out of the tapered hole which houses the tool and inserts the replacement tool. PA0 (2) The changer arm swivels the tool. PA0 (3) The tool in the spindle is clamped and unclamped. PA0 (4) The ATC protective cover is opened and closed. PA0 (5) The tool magazine is swivelled and indexed.
A tool changer according to the above cited Japanese Utility Model Application Laid-Open No. 63-113530 does not cover (3), (4) and (5) above. Both Japanese Utility Model Application Laid-Open No. 63-133945 and the Japanese Utility Model Publication No. 6-63833 do not cover (4) and (5). Therefore, with any of the above tool changers, the operations that are not covered by the tool changer have to be controlled independently by using one or more limit switches for detecting the end of the covered operations and/or the end of any of the independently controlled operations. Such limit switches are subject to breakage and can affect the reliability of the tool changer. Additionally, a limit switch requires time to confirm its proper functioning and this consequently prolongs the time required for changing tools. Finally, the use of independent motors is also required for any operations not covered by the tool changer, thus increasing the overall cost.